1. Field of the Invention
This invention relates in general to a heat insulating container and its manufacturing method, and more particularly to a heat insulating container having an inner glass container and an outer glass container.
2. Description of the Related Art
A heat insulating container comprising an inner glass container and an outer glass container has been in common use. The heat insulating container has a structure comprising an inner glass container is housed in an outer glass container with a gap formed between the inner and the outer glass containers. In this heat insulating container, the gap is a vacuum, or a low heat conductive gas (for example, argon, krypton, xenon) having a heat conductivity smaller than that of air is sealed within the gap, so that the gap constitutes a heat insulating layer. In this type of heat insulating container, a radiant heat preventive film is formed on an outer surface of the inner glass container and the inner surface of the outer glass container, wherein the radiant heat preventive film comprises silver and the like.
To manufacture thebove heat insulating container, for example, the following method may be adopted. The inner glass container and the outer glass container are integrally joined together on the ends of an opening. Next, a silver solution is injected into the gap such that the silver solution gets adhered to prescribed surfaces of the inner and outer glass container. Next, the resulting structure is heated at a temperature of about 200° C. to form a silver film. Subsequently, the gap is made vacuum by exhausting the gap of air through a tip tube provided on the outer container. Otherwise, a low heat conductivity gas is sealed within the gap. In both cases, the gap is transformed into a heat insulating layer.
Japanese Laid Open Publication No. 2001-505088 discloses a heat insulating container, in which a radiant heat preventive film made of metal oxide (SnO2, InO3, ZnO, etc.) with a resistivity of 10−4 Ω.cm is formed on the surfaces of the inner glass container and an outer glass container. The heat insulating container disclosed in thebove publication has an advantage that the contents of the container can be visible because the radiant heat preventive film is highly transparent.
Thebove radiant heat preventive film can be formed by using a sol-gel method which is described as follows. A metal complex solution is introduced into the gap such that the metal complex solution gets adhered to the prescribed surfaces of the inner and the outer glass containers. After that, the metal complex is dried under a heat treatment so that a metal oxide film is formed. For enhancing radiant heat insulating capability, the metal oxide film is reduced by subjecting it to a heat treatment, for example, by annealing at a temperature of about 400 to 600° C. in a reducing atmosphere, such as, in vacuum. Thus, the radiant heat preventive film made of metal oxide is obtained.
In thebove process, however, the thermal expansion coefficient of the inner and the outer glass containers differs widely from that of the radiant heat preventive film made of metal oxide. As a result, the shrinkage volume of the inner and the outer glass containers becomes widely different from that of the radiant heat preventive film when the heat insulating container is cooled down after thebove heat treatment. This shrinkage volume difference may cause an internal stress in the inner and outer glass containers. The stress resulting from the shrinkage of the radiant heat preventive film made of metal oxide is so strong that, for example, if a metal oxide film is formed on the surface of a thin-plated glass, the stress caused by the shrinkage of the metal oxide film could bend the thin-plated glass into a U-shape.
Besides, when the inner and the outer glass containers of above heat insulating container are manufactured (molded or transferred), a minute flaw such as crack and the like of several tens of nanometers to a few micrometer deep may be formed on the surface of the inner and outer glass containers. As a result, when the heat insulating container is cooled down after the heat treatment, the internal stress caused by thebove shrinkage volume difference becomes more concentrated on the flaw, which makes the inner and outer glass containers to easily break up from the flaw. Also, when the contents having a high temperature, such as hot water, is stored in the heat insulating container with such a flaw, an internal stress caused by thermal expansion acts on the flaw, causing the inner and outer glass containers, particularly the inner glass container, to easily break up.
Furthermore, when a glass containing a large amount of metal ions, such as soda glass, is used as the material making the inner and the outer glass containers, the metal ion (Na ion, K ion, Mg ion, etc.) may diffuse, and migrate into the radiant heat preventive film. In such a case, the resistivity of the radiant heat preventive film increases, while its carrier concentration decreases. As a result, the radiant heat preventive capacity of the radiant heat preventive film deteriorates. Yet another problem is that the radiant heat preventive film of a conventional heat insulating container (vacuum bottle) is made of silver, whose heat conductivity is high, which causes the conventional heat insulating container to lose part of its thermal insulating capability. This is because that when the radiant heat preventive film is provided near the end of the opening of the conventional heat insulating container, the heat in the container is transmitted through the radiant heat preventive film, allowing the heat to escape to outside due to the high heat conductivity of the radiant heat preventive film.